1. Field of the Invention
The present invention relates to a PN code generating apparatus applicable to a mobile device in a mobile communication system in which an intermittent reception is performed, for instance a CDMA (Code Division Multiple Access) communication system, and to a mobile radio communication system.
2. Description of the Related Art
Recently a mobile communication using a CDMA system has attracted public attention in a digital mobile communication field. In the U.S.A., the standardization of a mobile communication system in a CDMA system was performed by TIA (Telecommunications Industry Association), which is summarized in xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Digital Cellular Systemxe2x80x9d (IS-95-A) and so on.
In a CDMA system, transmission data are specturm spread with a spreading code that is different each channel. For instance, in IS-95-A includes a short PN code of 15th order (the period is about 26 ms) and a long PN code of 42th order (the period is about 41 days) consumed in spectrum spreading. And the long PN code is also used in the scramble for a forward link and in the assigning of an insert position of power control bit.
FIG. 1 illustrates a schematic configuration of a conventional PN code generating apparatus. As an example, the case of PN code of 42th order, which requires 42 delay elements in a shift register, is illustrated. PN code generating section 100 comprises a feedback shift register composed of 41 EX-ORs (EXclusive-OR circuit) 101, 42 one-clock-delay elements 102, 42 primitive polynomial coefficients g0 up to g41 and 42 multipliers 103.
In the PN code generating apparatus described above, the initial values of delay elements are set so that all values are not 0 at the same time, and the value of delay elements 102 is shifted corresponding to an input of shift clock 104 considering the feedback of the value of the last slot.
Any output of delay element is obtained as a PN code.
In a CDMA mobile communication system, a mobile device sets the initial value of delay element 102 at a system timing in the process of the synchronization acquisition with a base station, then generates a PN code using a chip rate consumed in spreading in a CDMA system as a shift clock.
A mobile device in a mobile communication system performs the monitoring reception to check a call once in the specific period predetermined with a base station while waiting. This is called an intermittent reception, in which as many circuits as possible except a timer for measuring a timing for the next monitoring reception are turned off during the non-reception period so as to reduce the consumed electric power.
However in a conventional PN code generating apparatus, since it is necessary to keep the synchronization of a code pattern of a long PN code with much longer period than the intermittent reception period even during the non-reception period, it is not possible to turn off the apparatus, which brought the problem that the reduction of electric power can not be achieved.
The present invention is carried out taking into account the above facts. The object of the present invention is to provide a PN code generating apparatus and method capable of acquiring the synchronization of long PN code immediately when the apparatus is restart after the stop state in an intermittent reception.
The first aspect of the present invention adopts the constitution comprising a PN code generating section for generating PN code has predetermined length using primitive polynomial G(x), then shifting the code content, and a state setting section for obtaining a code state of the PN code generating section after shifted the specific times from a code state of the PN code generating apparatus at a certain time, based on ximodG(x) as the number of shifts is i.
First ximodG(x) is calculated or obtained in advance where i is the number of shifts that should be necessary for a PN code generating apparatus in the case where it is assumed to be operating during the period from the turn-off to the next turn-on. The state at which the PN code generating apparatus is supposed to be when the next turn-on is obtained using ximodG(x), which indicates only the number of the shifts corresponding to the length of a PN code (42 shifts in this example) is enough to obtain the state of a PN code generating apparatus (the content of delay element) just after turned on using the state of the PN code generating apparatus (the content of delay element) just before turned off. Accordingly, it is possible to calculate the state of the PN code generating apparatus at the time of restarting a monitoring reception, while the PN code generating apparatus is turned off during the non-reception time and turned on just before the timing of the next monitoring reception. That allows to keep the PN code generating apparatus turn-off during almost of the non-reception time, which results in the reduction of the consumed electric current.
The second aspect of the present invention comprises a masking calculating section for acquiring the number of shifts: i corresponding to a period until a PN code generating apparatus restarts next to calculate ximodG(x).
It is possible to make a PN code generating apparatus the code state holding the synchronization with much less number of shifts than the number of shifts: i by providing i to a masking calculating section.
The third aspect of the present invention comprises a masking table in which a plurality of ximodG(x) obtained to a plurality of the number of shifts selected in advance are registered as a masking value, and a masking setting section for reading out the masking value from the masking table based on a value of n to obtain a status of a PN code generating apparatus after n*T time (n is an integer number) where the minimum period to calculate the target state of a PN code generating apparatus is T.
It is possible to obtain the state of a PN code generating apparatus after shifted the specific times from the state of the PN code generating apparatus at a certain time with only the number of shifts that is an integer times of the delay elements of the sift register.